Introducing myself and my project

[Shadowblues]

Hy there,

I have an SMA based solar power system, 9.9kWp. This is connected by 2 single phase inverters and a SMA Sunny Boy storage with BYD HV battery 9kWh. The system then is completed with an enwitec box that separates my internal system from the outside in case of blackout and gives me an island system with one phase (all three phases on one, so be careful with 3 phased loads).

I wanted to raise my battery power a bit but it was far to expensive and hard to deliver so I came to the DIY solution.

I am planning to integrate some 48V batteries to the system.
I want to build a 18650 battery - I have about 1300 cells lying around and testing them one by one. This will give me two batteries, each 14S30P or 14S60P - depends on the amount of batteries that I can get my hands on. All built with JKBMS and DIYBMS. Maybe one with that, one with the other, lets see.
And then I will also add a 16S battery with LiFePo as soon as I can get a reliable source for them

My problem is the integration in the SMA system. In normal operation, it should be no problem because if some problems occur, the power will just leave the system and destroy nothing. But whats in the case that all is separated in an island system? What inverter can I use to make that work and how to control that?

The main interrest of the expansion is to protect my main BYD battery which is integrated in the SMA system from dying too fast. So I want that the new battery will use the main load, this means - base load - about 800W in the night to 1500W during the day and to let the BYD battery just do all on top.

Thats the project.

A few works about me: 50 years, studied electronics, changed to software engineering and finished with Master degree. Since then I am working in the IT as an architect. Having 4 kids at home and my serverfarm as my test environment. Built two houses, made the electric cabling and KNX systems there. My house is running with IOBroker and Home assistant. Some things are monitored with KNX systems, other devices are controlled with Tasmota flashed ESP8266 plugs.

 

[OffGridInTheCity]

Welcome! Unfortunately, I don't know anything about SMA systems so I can't advise. But several here are actively running 18650 powerwalks and will be happy to share info.

From you're intro, I'm not sure if you have an existing battery as part of the SMA system. If so, can you share details such as type, voltage range, capacity, BMS if any. And pics are always fun

 

[Shadowblues]

From you're intro, I'm not sure if you have an existing battery as part of the SMA system. If so, can you share details such as type, voltage range, capacity, BMS if any. And pics are always fun

This is a BYD HV Battery. Type? BYD B-Box H9.0. Whats inside that black box? No plan? Whats the state of the batteries? No plan because no insight :-(

 

[OffGridInTheCity]

Found this link - http://baw-genergy.com/byd-battery-box-h9-0/
And it lists this bullet point....
Safe Battery Chemistry
B-Box is designed with LiFePO4 chemistry battery which has been widely recognized as one of the safest battery technologies. LiFePO4 chemistry features stable structures and its thermal runaway temperature is over 480°C. That’s 100% higher than NCM and NCA chemistry. It is designed for residential and commercial applications with absolute safety.

Suggests you have a LifePO4 battery. Be aware that 18650 (lithium-ion) is not so compatible because LifePo4 and Lithium-ion have significantly different charge/discharge curves. LifePO4 is very flat and Lithium-ion is more sloped.
- 16s LifePo4 = 3.65v * 16 = 58.4v max but rest at 3.45 * 16 = 55.2v.
- 14s Lithium-ion = 4.2v * 14 = 58.8v max but rest at 4.15 * 14 = 58.1v.
As an example, 55.2v LifePo4 rest = 55.2v/14 in series = 3.9v/cell for lithium-ion. This is safe, but you loose significant capacity between 3.9v and 4.1v / cell. If you try to go 13s you'll bump into a risk of overcharging the lithium-ion - which is unsafe.

All of which means that 18650 won't likely be a good way to expand your current battery. Can you get LifePo4 cells? Can you replace the existing battery completely with 18650? I don't know enough about SMA to advise in any direction here except that I wouldn't advise trying to mix LifePo4 with Lithium-ion.

As far as replacing the BYD completely. Do you have something like the pic below? Do you know the overall capacity in kwh?

A DIY 14s60p battery at 2,500ma/cell would be ~7kwh.

 

[Shadowblues]

Hi,

I don't want to replace the battery. I want to add another one with another inverter.
(Or add a second one to the Sunny Boy Storage system but I never found a compatible BMS system for a DIY battery. It would be high voltage anyway and a bit risky.)
And thats my problem - how to get the other inverter controlled in that system. Let us assume we take a Victron MP2.
The BYD battery should stay there as it is. Replacing the internals there will maybe be done in about 10 years.
At the moment I am trying to get enough EXTRA battery power to get it out of the 80% cycle every day.

But thank you. Now I know that I have a LiFePo battery in my garage Never looked that up.

But you brought up another interesting point. Lets assume we place a LiFePo and a LiIon battery in parallel and bring it to a Victron MP2 system. Lets also say that the load for both battery types would be very small - 0.5C on LiIon and 0.25C on LiFePo, so no battery comes in stress. Loading power is controlled in a way that both batteries can handle that.

Min and Max voltage points can be chosen in a way that both batteries can handle that since I only want to use them from about 20% to 80%.

A LiFePo battery have the long even voltage curve at 3.2V, the LiIon battery does have it at around 3.6V, so the complete batteries would have their stable voltage points at 16X3.2V = 51.2V, 14x3.6=50.4V. If now such a mixed battery out of two different types is discharged (or charged), the voltage will drop evenly on both batteries. At about 51.2V, most of the power will come from the LiFePo battery and this will change, so later most of the power will come from the LiIon battery. Why should this not work?